This invention relates generally to measuring liquid depth and particularly to measuring liquid levels in containers such as an underground tanks. This invention relates more particularly to measuring liquid levels in underground fuel tanks.
The principle needs expressed by users of the current wooden sticks are as follows: (1) The maximum cross section dimensions of the device must not exceed one inch. This permits it to be inserted into underground tanks that contain partially closed over-fill valves. This partially closed condition occurs as a tank nears full fill volume. Meeting this requirement with prior art devices such as that disclosed in U.S. Pat. No. 2,069,793 (Watson) would be difficult, if not impossible, to meet because the float mechanism is additive to the rectangular stick cross section. (2) The physical length of the device for most fuel delivery trucks must be limited to 14 feet, since this is the length of the transport storage tube that a truck/trailer can accommodate. However many large storage tanks are at least 12 feet in diameter, and may be buried over 4 feet below the surface. This requires that the “dip stick” be lowered into the tank by means of a lanyard. Watson's design is based on the use of a topside compression spring to assure that the float is locked in place. Testing of a single compression spring at the top of the float lock actuator has shown that the weight of the device was insufficient to overcome the required spring compression force to release the float. (3) Since the sticks in use today are subject to severe physical abuse, resulting in abrasion of all surfaces during insertion and withdrawal form the tanks and storage tubes, the measurement scales and the float position indicator must be recessed in such a manner to preclude contact with tank orifice lips, storage tube lips, and asphalt and concrete surfaces, (when the stick is laid horizontally on the ground). Current wood sticks only remain useful for short periods of time due to abrasive destruction of the scales and splintering. Watson's design shows the use of an exposed float position pointer, and a wooden stick with no means for protecting the scales from abrasion. (4) OSHA requires that any plastic or metal device used for insertion into the tanks be spark proof and not capable of producing a spark due to static electricity build up.